Interview with Prof. Lidia Morawska

Date: 5th Feb 2025

Topic: “Importance of ventilation and/or air cleaning in relation to risk of indoor transmission of infectious diseases”.

What are the key determinants that influence the spread of respiratory viruses through aerosols?

First, at least one infected person must be present in the space, carrying pathogens such as viruses or bacteria. Second, the infected person must share the space with others who are susceptible to infection. Third, the pathogens must accumulate in the environment, meaning they are not removed quickly enough to prevent others from inhaling a sufficient amount to cause infection. While concentration is an important factor, exposure time also plays a crucial role. For example, if viral particles build up in space but a person only spends five minutes there, such as when visiting a shop or supermarket, their exposure may be too brief to inhale enough pathogens for infection to occur. So, both concentration and the time you spend in this environment are effective factors. Broadly speaking, these are the prerequisites for infection.

What about transmission via hand-to-surface contact? That was a major focus at the beginning of the pandemic.

At the beginning of the pandemic, the primary discussion focused on surface transmission as the main route of infection. Studies have shown that the COVID-19 virus can be present on surfaces. However, no research has demonstrated that surface transmission was a major driver of infections. This doesn’t mean transmission via surfaces is impossible, but the probability is significantly lower compared to inhalation. That is the current state of knowledge. Even in hospitals, where some patients may sneeze, and their droplets land on nearby surfaces, such as bedside tables, this does not necessarily lead to infection. Medical staff do not constantly touch contaminated surfaces and then immediately bring their hands to their mouths. Therefore, the concern over surface transmission may be somewhat of a kind of artificial thinking.

Your point is that the main route of transmission is definitely through the air. The second thing that often comes up is the distinction between big and small droplets and the belief that most of the transmission happens through larger droplets. However, most of the smaller droplets, which are aerosols, are actually more relevant than people originally thought. Could you elaborate a bit more on this?

Well, here we are in the realm of terminology. Aerosols, particles, and related terms come from the scientific field. The definition of aerosols refers to solid and liquid particles suspended in a gaseous medium long enough to enable observation. Liquid particles are also called droplets. Therefore, there is no other division, particularly no division based on size. From that point of view, all particles emitted from human respiratory activities are droplets because they are liquid-based. However, we This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO).

can also call them aerosols since aerosols are liquid droplets. That’s why, in the scientific world pre-pandemic, we used these terms interchangeably.

It was at the beginning of the pandemic that I started realizing that we were speaking different languages, and I understood that there was a division. In the medical community, particles below 5 micrometres were considered aerosols, while those larger than 5 micrometres were classified separately, with different physical behaviours below and above this threshold. This distinction was potentially one of the reasons for not accepting airborne transmission. However, that didn’t help much, and I don’t think it was the main issue.

Since then, the WHO has issued a document and terminology document, and we are now talking about infectious respective particles, and that division of below 5 micrometres has been abolished. When discussing particles emitted from human respiratory activities, whether they carry a virus or not, the highest concentration is found in the smaller size range. The largest peak occurs below 1 micrometre, mainly originating from the deeper parts of the respiratory tract, particularly during breathing. A second peak, slightly larger in size, consists of particles generated from the larynx and upper respiratory tract. Beyond this, there is a much broader peak, though significantly lower in concentration, consisting of particles expelled from the mouth. These overlap with the second peak but extend into the visible size range. This is especially noticeable when someone is speaking excitedly, as large liquid particles can be seen being released.

And what does that range for the second peak?

This is two or three micrometres because the first one is just below 1 micrometre, and the other is just slightly larger. But they both overlap as well, so there's no sharp division between them.

Some medical people still believe that anything bigger than 5 micrometres is definitely not airborne and falls to the ground within seconds. Do you recognize that?

Well, if such arguments are made, I will bring an example from the outdoors. There was dust suspended in the middle of Australia from a drying lake, and we’re talking about a few thousand kilometres from here. The dust particles, which were in the range of over 10 micrometres, contributed to the PM10 concentration. The concentration of this dust here was so high that we suddenly couldn’t see across the river, and this dust had travelled a few thousand kilometres. So, it’s not just about gravitational force. If we had these particles in a vacuum, then only gravity would act on them, regardless of their size. However, in the air, apart from gravity, particles are also affected by flow dynamics and other forces. So, it is not gravity alone that immediately takes them down.

How do environmental factors like temperature, humidity, and external airflow impact the lifetime and spread of airborne droplets?

Well, the complexity here is that we start with a liquid particle emitted from our respiratory activities. This liquid particle is water-based, as water is its main component. However, it also contains various other substances such as proteins and salts, and if the person is infected, the virus. Initially, this particle exists in the high-humidity environment of the respiratory tract, which is close to 100%. Once emitted into the surrounding air, where the humidity is much lower, the particle immediately begins losing some of its water content. This process happens very quickly and, to some extent, depends This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO).

on the relative humidity of the environment. As a result, the particle shrinks, reducing its volume to about 20–40% of its original size. However, it does not lose all its water content; only a portion of it decreases.

So, when this happens, the evaporation process of that water is so rapid that it doesn't gradually occur as the particle moves farther away. It happens primarily within the first few centimetres, or tens of centimetres, from the source. After this initial phase, the majority of the evaporation has already occurred. Once this process takes place, the physical and chemical properties of the particle change. The concentration of whatever substances were in the particle becomes higher due to the reduced water content. Depending on the type of pathogen, this may either support its survival or hinder it. So, the effect on the pathogen's survival depends on the specific pathogen involved. There’s a significant variation, and I would say there’s still much we don't know. We understand the general mechanisms and what can happen, but predicting what happens to a specific particle in a particular room or environment is very difficult because we don't have all the parameters, either for the particle or the environment.

True, this differs from one virus to another and from one disease to another.

That's right. It changes depending on the virus, but we are still discovering new things. Maybe we knew these things, but not everyone was aware of them. For example, a study published last year by a group explored the impact of acidity on the virus, specifically COVID-19. So, where is this acidity coming from? The presence of CO2 in the air increases the acidity of the liquid droplet. This acidity helps the virus remain stable. Interestingly, this effect is not just at very high levels of CO2 concentration; it starts increasing the virus's stability from CO2 concentrations as low as 700-800 ppm, and we can see this effect up to concentrations of 1000 ppm and beyond. This is the critical range of CO2 concentrations that we need to control because, at this level, the risk of infection increases. However, it's not only that the risk of infection increases; CO2 concentration, as a proxy for infection transmission, also helps the virus remain more stable.

Some people believe that you don’t need to ventilate spaces extensively to supply fresh air; instead, you can address the issue with recirculating air cleaning solutions and different technologies. What do you think?

The issue is that ventilation should be the main focus. The key effort should be to provide proper ventilation, meaning fresh, clean air, whatever that definition of ‘fresh’ might be. When we consider naturally ventilated classrooms, such as in schools, like the majority of schools here in Australia, and as I hear in many other countries as well, it could be argued that, in this climate, most of the year, windows can be kept open, providing enough airflow. However, it's often considered either too hot or too cold in winter. Even so, winters here are not as extreme as in some other places, but it’s still seen as uncomfortable, perhaps too noisy, or even unsafe. There are many reasons why, as a result, many schools have windows closed by definition. So, what should be done in a situation where the building is naturally ventilated but it's not possible to provide enough ventilation? The focus should be on providing as much ventilation as possible. In that case, devices like air purifiers can help support the ventilation process, reducing the risk of infection. But it’s not an ideal situation, far from it, in my opinion. To maximize efforts in minimizing infection transmission, we should also consider other air quality risks as well. This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO).

Looking ahead, perhaps expecting another pandemic, if you had the budget to improve just 10% of global spaces, where would you prioritize those investments?

Well, everyone talks about the next pandemic, and people tend to think that because the previous one was 100 years ago, the next one won’t happen in their lifetime. So, they treat it as a purely hypothetical situation. But what people don’t realize is how many infections are happening all the time, how many kids, in particular, are constantly getting sick from all sorts of viruses and infections. What I’m stressing is: Let’s not worry so much about the next pandemic. Let’s focus on the current epidemics and do our best to reduce the risk of infection from the known pathogens we’re dealing with now.

What kind of diseases are we talking about?

Colds, flu, chickenpox, measles, basically anything that is respiratory. There is a very large range of different kinds of infections, including gastroenteritis and others. When we compared the infectiousness of different pathogens, we first looked at this in 2021. At that time, COVID-19 was still the wild type, which was mid-range in terms of infectiousness. Measles was the most infectious disease, but when we looked at COVID-19, it surpassed all the known pathogens in terms of infectiousness. I imagine that we haven't done this for the currently circulating variants, but I imagine COVID-19 is still the most infectious disease. The point is that no one wants to hear about this. So, when we talk about indoor air quality projects here in Australia, even though COVID is mentioned, it often becomes a lost cause. People don't want to hear about COVID anymore; it's like it's gone, and they don’t want to engage with this very interesting approach.

What combination of technological solutions, like the ventilation system, air treatment, face masks, etc, is most effective for mitigating the spread of urban viruses?

Well, all of these technologies, but not necessarily all of them at the same time, depend very much on the situation. So let's start with a situation at a conference venue that is very well-ventilated. In particular, the venue in Rotterdam, where we were, was very well-ventilated, with very low concentrations of CO2, which I measured. In situations like this, the probability of infection from attending the sessions is relatively low at the conference. But still, there is the issue of close proximity. While we don't expect a large number of participants to get infected, the close proximity remains a problem. So, if you were in a meeting at that conference or talking to others, wearing a mask makes sense. However, if you're on a break and having a coffee, wearing the mask becomes a bit of a problem. These social situations make it a bit difficult. What surprised me, I must say, was that in Rotterdam and later during the pandemic, people didn’t seem to think about close proximity. I would automatically take two or three steps back. As we’ve calculated, how close you stay and how long you talk increases the risk of infection. So, this is the situation in a well-ventilated space with a mask. But then, from there, you go to many other situations, particularly those where ventilation is not particularly good, and there are plenty of examples of this.

But then the question is: all right, how do you know that, and what do you do? Is it something you do as an individual, or is it something that whoever is in charge of the building should do? Still, the question is: what is the ventilation? Do I know what the ventilation is? Well, I can measure it with a This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO).

CO2 meter. But if I don't measure, and if the place looks crowded, maybe it’s not good. So then, the next step is: if it's not good and I measure it, what do I do?

There are other devices you can use, and one of these technologies is filtration using purifiers. Now, air purifiers are a very good technology, and I'm talking about filter-based air purifiers. While it’s a very simple physical process behind them, it is also effective. The air goes over the filters repeatedly and very efficiently reduces the concentration of viruses in the air.

You're talking about systems that are in the room, circulating the air and filtering it, not something centrally located in the HVAC system?

We covered that solution in central systems. If there is good ventilation, usually in a mechanically ventilated building, there is also filtration, which is the norm. But when we move to a situation where the ventilation is not good, whether in a mechanically ventilated building or a naturally ventilated one, then we have to rely on standalone air purifiers.

Do you mean like mobile ones?

Which is the setting. Yeah, they are typically mobile. The benefit of having a device like this is that it can also be used at installation when you have particular outdoor air pollution. So, it removes these particles as well. One of the issues with this, and what I've seen happening during this pandemic, is, for example, there was a big project in the state of Victoria when, in September 2021, the government announced a big program, a 200-million-dollar initiative, to start buying air purifiers and giving them to school classrooms. My immediate reaction to this was, well, that's very good. But are they going to be used? And I have to check when the first presentation was in which I said this would result in a lot of unused electronics. And a few years later, that's what's happened. Now, why is it electronic junk, and why haven't they been used? First of all, if you provide a device to somebody, like a teacher who's busy all the time, and says, "Well, that's the box that does something about this," unless the teacher is really enthusiastic and has some knowledge about it, it just becomes another burden. "Do I need to do this?" Well, let's leave it standing. But even if it's turned on and if it's noisy, and many of them are noisy because they use HEPA filters, which require a lot of pressure, if it's noisy and I can't have the kids hear me when I speak, I have to turn it down or off. In many cases, that was a big issue. So, there is a lot of room for improving these purifiers in terms of equipping them, particularly with HEPA filters that are less efficient but still good filters with lower pressure. That would do the same job but with less noise. So that's one solution.

So, do you mean electrostatic filters, for example, instead of HEPA filters?

Well, even lower-grade material filters. We've done some measurements years ago in one of the office buildings. We mentioned it in one of the papers. We compared the operation of different filters in different places of this building, and we showed that lower-grade filters were doing an equally good job because a good part of the areas were circulated.

We also have some research that findings show at least not all of them, but some of them were not being used, especially because of the noise.

Yeah, so actually, within our programmes, one of the projects is focused on this, and the idea is, first of all, to test performance, including performance with... but also to have them operating This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO).

automatically, such that they turn themselves on when the concentration of CO2 or PM2.5 exceeds the set levels and turn off when necessary. This way, they could be placed in a spot where they are out of the way, operating without any involvement from a teacher or anybody.

What do you think of the idea of having it in a waiting room of a hospital or a doctor's office, mounted on the wall so it's not in the way? Then, the noise issue will be less of a problem if it's well-installed, etc.

So this is one of the best technologies and the easiest to use. Now, another type of technology is the GUV. GUV is also very efficient in deactivating viruses. However, compared to filter-based purifiers, it does only one thing: deactivate viruses. It doesn’t matter the particle size. So, let's say if I had a choice, if I were a teacher or whatever, and I had to make a decision, knowing that they would be pushed by us all the time, what would I choose? I would choose an air purifier because it does both. But, having said that, GUV is a good technology. Still, there’s more complexity to this because if we’re talking about upper-room GUV, the issue is that we still can’t completely get away from ventilation. We still need to have proper ventilation in place. We’ve got to be very conscious of radiation risks and consider all kinds of potential situations where someone could be irradiated. So, from that perspective, it’s much more complex. The technology is less problematic in terms of potential radiation exposure, so from that point of view, you can place it where people are. I’ve seen some devices placed between people in a meeting, for example, on the table. So, from that point of view, it's potentially better. There's still the consideration of the potential for forming secondary products. Some proponents strongly argue that secondary products are formed. We have a paper under review that I can't say much about, but in real situations, we didn’t show that much of a risk. The problem is that if you do measurements of the secondary products in a laboratory setting or a chamber, you don't know what the real situations are like. Therefore, it’s easier to predict what might happen in such environments, and most claims about these potentially high levels of secondary products have been based on this kind of modelling. That said, of course, it's possible, but in real environments, what we've seen suggests that there is much less risk. However, there have not been enough studies done in real environments to definitively prove whether this is a big risk.

Also, when measuring air quality, it's important to consider what's happening outdoors. You might look at a time series of air quality data, notice an increase in certain pollutants, and immediately assume that the device is responsible. But if you also check outdoor air conditions, you might find that pollution levels are rising there, too. So, without understanding the interaction between indoor and outdoor air, it’s easy to misinterpret the data. A good example of this comes from an experiment I wrote about in a commentary for Atmospheric Environment, titled Not to Peel an Orange. We conducted the experiment in an office setting. My research assistant was sitting at her desk, and we placed a particle monitor next to her. Initially, the air had about 10.000 particles per cubic centimetre. Then she started peeling an orange, and within two or three minutes, the particle count increased rapidly to 30.000. If you didn’t know the cause, you might assume something in the room, like an air purifier, was responsible for the increase. But in reality, it was just the simple act of peeling an orange. Now, imagine if a GUV device had been running at the same time. People might have wrongly blamed it for the spike in particles, even though it had nothing to do with it. The key takeaway is that many factors can influence air quality, and we need to be careful when attributing changes to a specific source without a full understanding of the context. This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO).

What are your top recommendations for people to maintain safe indoor air quality, especially in shared spaces?

This question has two elements. Are we talking about people and individual responsibility, or are we talking about operators of the buildings? This is the responsibility of the operators because this is ultimately when we are thinking about the future; it shouldn't be our individual responsibility to do things to improve air quality or, of course, not to bring pollution sources. But it should be part of the building operation that air is set. But right now, it's still not the case, and we are in this transition period and hopefully closer to that transition being completed. So we need both: we need the operators to be aware of the problem and do something, and we need to, as individuals, put pressure on and protect ourselves.

So, what to do? In a way, it is simple, but it's also not that simple to me. It is simple because I have a professional background in this. So, I walk into the space, and even though I'm not a building engineer, I can't tell that the air is coming from here, but I have a general idea of what's happening in this space. But I'm also working all the time with my handbag, which is my airnet. So, all the time, I measure the concentration, and depending on the situation and the reading, I then adapt my behaviour and my actions. If the concentration of CO2 is high, and I have to stay in that space for whatever reason for a sufficient period of time, I put on a mask, which is also in my handbag.

What kind of what kind of masks?

We've done some measurements using mask wear before the pandemic, and that was related to our work with cystic fibrosis patients and Pseudomonas bacteria, which was a focus of our research. We had a tunnel in which we measured this, and for those people, we compared N95 masks and surgical masks. So, in general, we showed that surgical masks, for that situation, were effective, but the focus was on emissions, not necessarily infection. So, our conclusion was that both types of masks were comparable. The most important factor is how they are fitted. It is easier to fit an N95 mask better than a surgical mask. So, usually, in installation situations, particularly with longer exposure, the probabilities are higher. Therefore, personally, I prefer the N95 mask. But if you're not spending too much time somewhere, a surgical mask will work as well.

This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO). Interview with Prof. Lidia Morawska Date: 5th Feb 2025 Topic: “Importance of ventilation and/or air cleaning in relation to risk of indoor transmission of infectious diseases”. What are the key determinants that influence the spread of respiratory viruses through aerosols? First, at least one infected person must be present in the space, carrying pathogens such as viruses or bacteria. Second, the infected person must share the space with others who are susceptible to infection. Third, the pathogens must accumulate in the environment, meaning they are not removed quickly enough to prevent others from inhaling a sufficient amount to cause infection. While concentration is an important factor, exposure time also plays a crucial role. For example, if viral particles build up in space but a person only spends five minutes there, such as when visiting a shop or supermarket, their exposure may be too brief to inhale enough pathogens for infection to occur. So, both concentration and the time you spend in this environment are effective factors. Broadly speaking, these are the prerequisites for infection. What about transmission via hand-to-surface contact? That was a major focus at the beginning of the pandemic. At the beginning of the pandemic, the primary discussion focused on surface transmission as the main route of infection. Studies have shown that the COVID-19 virus can be present on surfaces. However, no research has demonstrated that surface transmission was a major driver of infections. This doesn’t mean transmission via surfaces is impossible, but the probability is significantly lower compared to inhalation. That is the current state of knowledge. Even in hospitals, where some patients may sneeze, and their droplets land on nearby surfaces, such as bedside tables, this does not necessarily lead to infection. Medical staff do not constantly touch contaminated surfaces and then immediately bring their hands to their mouths. Therefore, the concern over surface transmission may be somewhat of a kind of artificial thinking. Your point is that the main route of transmission is definitely through the air. The second thing that often comes up is the distinction between big and small droplets and the belief that most of the transmission happens through larger droplets. However, most of the smaller droplets, which are aerosols, are actually more relevant than people originally thought. Could you elaborate a bit more on this? Well, here we are in the realm of terminology. Aerosols, particles, and related terms come from the scientific field. The definition of aerosols refers to solid and liquid particles suspended in a gaseous medium long enough to enable observation. Liquid particles are also called droplets. Therefore, there is no other division, particularly no division based on size. From that point of view, all particles emitted from human respiratory activities are droplets because they are liquid-based. However, we This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO). can also call them aerosols since aerosols are liquid droplets. That’s why, in the scientific world pre-pandemic, we used these terms interchangeably. It was at the beginning of the pandemic that I started realizing that we were speaking different languages, and I understood that there was a division. In the medical community, particles below 5 micrometres were considered aerosols, while those larger than 5 micrometres were classified separately, with different physical behaviours below and above this threshold. This distinction was potentially one of the reasons for not accepting airborne transmission. However, that didn’t help much, and I don’t think it was the main issue. Since then, the WHO has issued a document and terminology document, and we are now talking about infectious respective particles, and that division of below 5 micrometres has been abolished. When discussing particles emitted from human respiratory activities, whether they carry a virus or not, the highest concentration is found in the smaller size range. The largest peak occurs below 1 micrometre, mainly originating from the deeper parts of the respiratory tract, particularly during breathing. A second peak, slightly larger in size, consists of particles generated from the larynx and upper respiratory tract. Beyond this, there is a much broader peak, though significantly lower in concentration, consisting of particles expelled from the mouth. These overlap with the second peak but extend into the visible size range. This is especially noticeable when someone is speaking excitedly, as large liquid particles can be seen being released. And what does that range for the second peak? This is two or three micrometres because the first one is just below 1 micrometre, and the other is just slightly larger. But they both overlap as well, so there's no sharp division between them. Some medical people still believe that anything bigger than 5 micrometres is definitely not airborne and falls to the ground within seconds. Do you recognize that? Well, if such arguments are made, I will bring an example from the outdoors. There was dust suspended in the middle of Australia from a drying lake, and we’re talking about a few thousand kilometres from here. The dust particles, which were in the range of over 10 micrometres, contributed to the PM10 concentration. The concentration of this dust here was so high that we suddenly couldn’t see across the river, and this dust had travelled a few thousand kilometres. So, it’s not just about gravitational force. If we had these particles in a vacuum, then only gravity would act on them, regardless of their size. However, in the air, apart from gravity, particles are also affected by flow dynamics and other forces. So, it is not gravity alone that immediately takes them down. How do environmental factors like temperature, humidity, and external airflow impact the lifetime and spread of airborne droplets? Well, the complexity here is that we start with a liquid particle emitted from our respiratory activities. This liquid particle is water-based, as water is its main component. However, it also contains various other substances such as proteins and salts, and if the person is infected, the virus. Initially, this particle exists in the high-humidity environment of the respiratory tract, which is close to 100%. Once emitted into the surrounding air, where the humidity is much lower, the particle immediately begins losing some of its water content. This process happens very quickly and, to some extent, depends This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO). on the relative humidity of the environment. As a result, the particle shrinks, reducing its volume to about 20–40% of its original size. However, it does not lose all its water content; only a portion of it decreases. So, when this happens, the evaporation process of that water is so rapid that it doesn't gradually occur as the particle moves farther away. It happens primarily within the first few centimetres, or tens of centimetres, from the source. After this initial phase, the majority of the evaporation has already occurred. Once this process takes place, the physical and chemical properties of the particle change. The concentration of whatever substances were in the particle becomes higher due to the reduced water content. Depending on the type of pathogen, this may either support its survival or hinder it. So, the effect on the pathogen's survival depends on the specific pathogen involved. There’s a significant variation, and I would say there’s still much we don't know. We understand the general mechanisms and what can happen, but predicting what happens to a specific particle in a particular room or environment is very difficult because we don't have all the parameters, either for the particle or the environment. True, this differs from one virus to another and from one disease to another. That's right. It changes depending on the virus, but we are still discovering new things. Maybe we knew these things, but not everyone was aware of them. For example, a study published last year by a group explored the impact of acidity on the virus, specifically COVID-19. So, where is this acidity coming from? The presence of CO2 in the air increases the acidity of the liquid droplet. This acidity helps the virus remain stable. Interestingly, this effect is not just at very high levels of CO2 concentration; it starts increasing the virus's stability from CO2 concentrations as low as 700-800 ppm, and we can see this effect up to concentrations of 1000 ppm and beyond. This is the critical range of CO2 concentrations that we need to control because, at this level, the risk of infection increases. However, it's not only that the risk of infection increases; CO2 concentration, as a proxy for infection transmission, also helps the virus remain more stable. Some people believe that you don’t need to ventilate spaces extensively to supply fresh air; instead, you can address the issue with recirculating air cleaning solutions and different technologies. What do you think? The issue is that ventilation should be the main focus. The key effort should be to provide proper ventilation, meaning fresh, clean air, whatever that definition of ‘fresh’ might be. When we consider naturally ventilated classrooms, such as in schools, like the majority of schools here in Australia, and as I hear in many other countries as well, it could be argued that, in this climate, most of the year, windows can be kept open, providing enough airflow. However, it's often considered either too hot or too cold in winter. Even so, winters here are not as extreme as in some other places, but it’s still seen as uncomfortable, perhaps too noisy, or even unsafe. There are many reasons why, as a result, many schools have windows closed by definition. So, what should be done in a situation where the building is naturally ventilated but it's not possible to provide enough ventilation? The focus should be on providing as much ventilation as possible. In that case, devices like air purifiers can help support the ventilation process, reducing the risk of infection. But it’s not an ideal situation, far from it, in my opinion. To maximize efforts in minimizing infection transmission, we should also consider other air quality risks as well. This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO). Looking ahead, perhaps expecting another pandemic, if you had the budget to improve just 10% of global spaces, where would you prioritize those investments? Well, everyone talks about the next pandemic, and people tend to think that because the previous one was 100 years ago, the next one won’t happen in their lifetime. So, they treat it as a purely hypothetical situation. But what people don’t realize is how many infections are happening all the time, how many kids, in particular, are constantly getting sick from all sorts of viruses and infections. What I’m stressing is: Let’s not worry so much about the next pandemic. Let’s focus on the current epidemics and do our best to reduce the risk of infection from the known pathogens we’re dealing with now. What kind of diseases are we talking about? Colds, flu, chickenpox, measles, basically anything that is respiratory. There is a very large range of different kinds of infections, including gastroenteritis and others. When we compared the infectiousness of different pathogens, we first looked at this in 2021. At that time, COVID-19 was still the wild type, which was mid-range in terms of infectiousness. Measles was the most infectious disease, but when we looked at COVID-19, it surpassed all the known pathogens in terms of infectiousness. I imagine that we haven't done this for the currently circulating variants, but I imagine COVID-19 is still the most infectious disease. The point is that no one wants to hear about this. So, when we talk about indoor air quality projects here in Australia, even though COVID is mentioned, it often becomes a lost cause. People don't want to hear about COVID anymore; it's like it's gone, and they don’t want to engage with this very interesting approach. What combination of technological solutions, like the ventilation system, air treatment, face masks, etc, is most effective for mitigating the spread of urban viruses? Well, all of these technologies, but not necessarily all of them at the same time, depend very much on the situation. So let's start with a situation at a conference venue that is very well-ventilated. In particular, the venue in Rotterdam, where we were, was very well-ventilated, with very low concentrations of CO2, which I measured. In situations like this, the probability of infection from attending the sessions is relatively low at the conference. But still, there is the issue of close proximity. While we don't expect a large number of participants to get infected, the close proximity remains a problem. So, if you were in a meeting at that conference or talking to others, wearing a mask makes sense. However, if you're on a break and having a coffee, wearing the mask becomes a bit of a problem. These social situations make it a bit difficult. What surprised me, I must say, was that in Rotterdam and later during the pandemic, people didn’t seem to think about close proximity. I would automatically take two or three steps back. As we’ve calculated, how close you stay and how long you talk increases the risk of infection. So, this is the situation in a well-ventilated space with a mask. But then, from there, you go to many other situations, particularly those where ventilation is not particularly good, and there are plenty of examples of this. But then the question is: all right, how do you know that, and what do you do? Is it something you do as an individual, or is it something that whoever is in charge of the building should do? Still, the question is: what is the ventilation? Do I know what the ventilation is? Well, I can measure it with a This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO). CO2 meter. But if I don't measure, and if the place looks crowded, maybe it’s not good. So then, the next step is: if it's not good and I measure it, what do I do? There are other devices you can use, and one of these technologies is filtration using purifiers. Now, air purifiers are a very good technology, and I'm talking about filter-based air purifiers. While it’s a very simple physical process behind them, it is also effective. The air goes over the filters repeatedly and very efficiently reduces the concentration of viruses in the air. You're talking about systems that are in the room, circulating the air and filtering it, not something centrally located in the HVAC system? We covered that solution in central systems. If there is good ventilation, usually in a mechanically ventilated building, there is also filtration, which is the norm. But when we move to a situation where the ventilation is not good, whether in a mechanically ventilated building or a naturally ventilated one, then we have to rely on standalone air purifiers. Do you mean like mobile ones? Which is the setting. Yeah, they are typically mobile. The benefit of having a device like this is that it can also be used at installation when you have particular outdoor air pollution. So, it removes these particles as well. One of the issues with this, and what I've seen happening during this pandemic, is, for example, there was a big project in the state of Victoria when, in September 2021, the government announced a big program, a 200-million-dollar initiative, to start buying air purifiers and giving them to school classrooms. My immediate reaction to this was, well, that's very good. But are they going to be used? And I have to check when the first presentation was in which I said this would result in a lot of unused electronics. And a few years later, that's what's happened. Now, why is it electronic junk, and why haven't they been used? First of all, if you provide a device to somebody, like a teacher who's busy all the time, and says, "Well, that's the box that does something about this," unless the teacher is really enthusiastic and has some knowledge about it, it just becomes another burden. "Do I need to do this?" Well, let's leave it standing. But even if it's turned on and if it's noisy, and many of them are noisy because they use HEPA filters, which require a lot of pressure, if it's noisy and I can't have the kids hear me when I speak, I have to turn it down or off. In many cases, that was a big issue. So, there is a lot of room for improving these purifiers in terms of equipping them, particularly with HEPA filters that are less efficient but still good filters with lower pressure. That would do the same job but with less noise. So that's one solution. So, do you mean electrostatic filters, for example, instead of HEPA filters? Well, even lower-grade material filters. We've done some measurements years ago in one of the office buildings. We mentioned it in one of the papers. We compared the operation of different filters in different places of this building, and we showed that lower-grade filters were doing an equally good job because a good part of the areas were circulated. We also have some research that findings show at least not all of them, but some of them were not being used, especially because of the noise. Yeah, so actually, within our programmes, one of the projects is focused on this, and the idea is, first of all, to test performance, including performance with... but also to have them operating This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO). automatically, such that they turn themselves on when the concentration of CO2 or PM2.5 exceeds the set levels and turn off when necessary. This way, they could be placed in a spot where they are out of the way, operating without any involvement from a teacher or anybody. What do you think of the idea of having it in a waiting room of a hospital or a doctor's office, mounted on the wall so it's not in the way? Then, the noise issue will be less of a problem if it's well-installed, etc. So this is one of the best technologies and the easiest to use. Now, another type of technology is the GUV. GUV is also very efficient in deactivating viruses. However, compared to filter-based purifiers, it does only one thing: deactivate viruses. It doesn’t matter the particle size. So, let's say if I had a choice, if I were a teacher or whatever, and I had to make a decision, knowing that they would be pushed by us all the time, what would I choose? I would choose an air purifier because it does both. But, having said that, GUV is a good technology. Still, there’s more complexity to this because if we’re talking about upper-room GUV, the issue is that we still can’t completely get away from ventilation. We still need to have proper ventilation in place. We’ve got to be very conscious of radiation risks and consider all kinds of potential situations where someone could be irradiated. So, from that perspective, it’s much more complex. The technology is less problematic in terms of potential radiation exposure, so from that point of view, you can place it where people are. I’ve seen some devices placed between people in a meeting, for example, on the table. So, from that point of view, it's potentially better. There's still the consideration of the potential for forming secondary products. Some proponents strongly argue that secondary products are formed. We have a paper under review that I can't say much about, but in real situations, we didn’t show that much of a risk. The problem is that if you do measurements of the secondary products in a laboratory setting or a chamber, you don't know what the real situations are like. Therefore, it’s easier to predict what might happen in such environments, and most claims about these potentially high levels of secondary products have been based on this kind of modelling. That said, of course, it's possible, but in real environments, what we've seen suggests that there is much less risk. However, there have not been enough studies done in real environments to definitively prove whether this is a big risk. Also, when measuring air quality, it's important to consider what's happening outdoors. You might look at a time series of air quality data, notice an increase in certain pollutants, and immediately assume that the device is responsible. But if you also check outdoor air conditions, you might find that pollution levels are rising there, too. So, without understanding the interaction between indoor and outdoor air, it’s easy to misinterpret the data. A good example of this comes from an experiment I wrote about in a commentary for Atmospheric Environment, titled Not to Peel an Orange. We conducted the experiment in an office setting. My research assistant was sitting at her desk, and we placed a particle monitor next to her. Initially, the air had about 10.000 particles per cubic centimetre. Then she started peeling an orange, and within two or three minutes, the particle count increased rapidly to 30.000. If you didn’t know the cause, you might assume something in the room, like an air purifier, was responsible for the increase. But in reality, it was just the simple act of peeling an orange. Now, imagine if a GUV device had been running at the same time. People might have wrongly blamed it for the spike in particles, even though it had nothing to do with it. The key takeaway is that many factors can influence air quality, and we need to be careful when attributing changes to a specific source without a full understanding of the context. This interview is part of the MIST project with project number P20-35 of the research programme Perspectief, which is financed by the Dutch Research Council (NWO). What are your top recommendations for people to maintain safe indoor air quality, especially in shared spaces? This question has two elements. Are we talking about people and individual responsibility, or are we talking about operators of the buildings? This is the responsibility of the operators because this is ultimately when we are thinking about the future; it shouldn't be our individual responsibility to do things to improve air quality or, of course, not to bring pollution sources. But it should be part of the building operation that air is set. But right now, it's still not the case, and we are in this transition period and hopefully closer to that transition being completed. So we need both: we need the operators to be aware of the problem and do something, and we need to, as individuals, put pressure on and protect ourselves. So, what to do? In a way, it is simple, but it's also not that simple to me. It is simple because I have a professional background in this. So, I walk into the space, and even though I'm not a building engineer, I can't tell that the air is coming from here, but I have a general idea of what's happening in this space. But I'm also working all the time with my handbag, which is my airnet. So, all the time, I measure the concentration, and depending on the situation and the reading, I then adapt my behaviour and my actions. If the concentration of CO2 is high, and I have to stay in that space for whatever reason for a sufficient period of time, I put on a mask, which is also in my handbag. What kind of what kind of masks? We've done some measurements using mask wear before the pandemic, and that was related to our work with cystic fibrosis patients and Pseudomonas bacteria, which was a focus of our research. We had a tunnel in which we measured this, and for those people, we compared N95 masks and surgical masks. So, in general, we showed that surgical masks, for that situation, were effective, but the focus was on emissions, not necessarily infection. So, our conclusion was that both types of masks were comparable. The most important factor is how they are fitted. It is easier to fit an N95 mask better than a surgical mask. So, usually, in installation situations, particularly with longer exposure, the probabilities are higher. Therefore, personally, I prefer the N95 mask. But if you're not spending too much time somewhere, a surgical mask will work as well. So, this is an aspect of protecting myself and taking action on my own behalf. I can't resist doing things wherever I see the concentrations of CO2 and ventilation issues that are problematic., this is an aspect of protecting myself and taking action on my own behalf. I can't resist doing things wherever I see the concentrations of CO2 and ventilation issues that are problematic.